Conventionally, a solvent deasphalting (SDA) process is employed by an oil refinery for the purpose of extracting valuable components from a residual oil feedstock, which is a heavy hydrocarbon that is produced as a by-product of refining crude oil. The extracted components are fed back to the refinery wherein they are converted into valuable lighter fractions such as gasoline. Suitable residual oil feedstocks which may be used in a SDA process include, for example, atmospheric tower bottoms, vacuum tower bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands.
In a typical SDA process, a light hydrocarbon solvent is added to the residual oil feed from a refinery and is processed in what can be termed as an asphaltene separator. Common solvents used comprise light paraffinic solvents. Examples of light paraffinic solvents include, but are not limited to, propane, butane, isobutane, pentane, isopentane, neopentane, hexane, isohexane, heptane, and similar known solvents used in deasphalting, and mixtures thereof. Under elevated temperature and pressures, the mixture in the asphaltene separator separates into a plurality of liquid streams, typically, a substantially asphaltene-free stream of deasphalted oil (DAO), resins and solvent, and a mixture of asphaltene and solvent within which some DAO may be dissolved.
Once the asphaltenes have been removed, the substantially asphaltene-free stream of DAO, resins and solvent is normally subjected to a solvent recovery system. The solvent recovery system of an SDA unit extracts a fraction of the solvent from the solvent rich DAO by boiling off the solvent, commonly using steam or hot oil from fired heaters. The vaporized solvent is then condensed and recycled back for use in the SDA unit.
Often it becomes beneficial to separate a resin product from the DAO/resin product stream. This is normally done before the solvent is removed from the DAO. “Resins” as used herein, means resins that have been separated and obtained from a SDA unit. Resins are denser or heavier than deasphalted oil, but lighter than the aforementioned asphaltenes. The resin product usually comprises more aromatic hydrocarbons with highly aliphatic substituted side chains, and can also comprise metals, such as nickel and vanadium. Generally, the resins comprise the material from which asphaltenes and DAO have been removed.
Crude oils contain heteroatomic, polyaromatic molecules that include compounds such as sulfur, nitrogen, nickel, vanadium and others in quantities that can adversely affect the refinery processing of crude oil fractions. Light crude oils or condensates have sulfur concentrations as low as 0.01 percent by weight (W %). In contrast, heavy crude oils and heavy petroleum fractions have sulfur concentrations as high as 5-6 W %. Similarly, the nitrogen content of crude oils can be in the range of 0.001-1.0 W %. These impurities must be removed during refining to meet established environmental regulations for the final products (e.g., gasoline, diesel, fuel oil), or for the intermediate refining streams that are to be processed for further upgrading, such as isomerization or reforming. Furthermore, contaminants such as nitrogen, sulfur and heavy metals are known to deactivate or poison catalysts, and thus must be removed.
Asphaltenes, which are solid in nature and comprise polynuclear aromatics present in the solution of smaller aromatics and resin molecules, are also present in the crude oils and heavy fractions in varying quantities. Asphaltenes do not exist in all of the condensates or in light crude oils; however, they are present in relatively large quantities in heavy crude oils and petroleum fractions. Asphaltenes are insoluble components or fractions and their concentrations are defined as the amount of asphaltenes precipitated by addition of an n-paraffin solvent to the feedstock.
In a typical refinery, crude oil is first fractionated in the atmospheric distillation column to separate sour gas including methane, ethane, propanes, butanes and hydrogen sulfide, naphtha (boiling point range: 36-180° C.), kerosene (boiling point range: 180-240° C.), gas oil (boiling point range: 240-370° C.) and atmospheric residue, which are the hydrocarbon fractions boiling above 370° C. The atmospheric residue from the atmospheric distillation column is either used as fuel oil or sent to a vacuum distillation unit, depending upon the configuration of the refinery. Principal products from the vacuum distillation are vacuum gas oil, comprising hydrocarbons boiling in the range 370-520° C., and vacuum residue, comprising hydrocarbons boiling above 520° C.
Naphtha, kerosene and gas oil streams derived from crude oils or other natural sources, such as shale oils, bitumens and tar sands, are treated to remove the contaminants, such as sulfur, that exceed the specification set for the end product(s). Hydrotreating is the most common refining technology used to remove these contaminants. Vacuum gas oil is processed in a hydrocracking unit to produce gasoline and diesel, or in a fluid catalytic cracking (FCC) unit to produce mainly gasoline, light cycle oil (LCO) and heavy cycle oil (HCO) as by-products, the former being used as a blending component in either the diesel pool or in fuel oil, the latter being sent directly to the fuel oil pool.
There are several processing options for the vacuum residue fraction, including hydroprocessing (including both residue hydrotreating and residue hydrocracking which includes both ebullated bed and slurry phase type reactors), coking, visbreaking, gasification and solvent deasphalting. Solvent deasphalting (SDA) is a well proven technology for separation of residues by their molecular weight and is practiced commercially worldwide. The separation in the SDA process can be into two or sometimes three components, i.e., a two component SDA process or a three component SDA process. In the SDA process, the asphaltenes rich fraction (pitch) comprising about 6-8 W % of hydrogen is separated from the vacuum residue by contact with a paraffinic solvent (carbon number ranging from 3-8) at elevated temperatures and pressures. The recovered deasphalted oil fraction (DAO) comprising about 9-11 W % hydrogen, is characterized as a heavy hydrocarbon fraction that is free of asphaltene molecules and can be sent to other conversion units such as a hydroprocessing unit or a fluid catalytic cracking unit (FCC) for further processing.
The yield of DAO is usually set by the processing feed stock property limitations, such as organometallic metals and Conradson Carbon residue (CCR) of the downstream processes. These limitations are usually below the maximum recoverable DAO within the SDA process (Table 1 and FIG. 1). Table 1 illustrates typical yields obtained in a SDA process. If the DAO yield can be increased, then the overall valuable transportation fuel yields, based on residue feed, can be increased, and the profitability of SDA enhanced. A parallel benefit would occur with the combination of SDA followed by delayed coking. Maximizing DAO yield maximizes the catalytic conversion of residue relative to thermal conversion, which occurs in delayed coking.
TABLE 1DAOFEED(HC limited)PITCHVOL-%100.0053.2146.79WEIGHT-%100.0050.0050.00API5.3714.2−3.4Sp. Gr.1.03380.97151.1047S, wt-%4.273.035.51N, wppm0.300Con Carbon, wt-%237.738.3C7 insols, wt-%6.860.0513.7UOP K11.2711.5411.01Ni, ppm242.046.0V, ppm945.2182.8
Even without DAO downstream processing limitations, the cost of hydroprocessing DAO can be very high. In examining the DAO properties and its composition (Table 2), it can be seen that the back end of DAO, typically referred to as the Resin fraction, sets the severity and ultimately cost of the hydroprocessing unit. It would therefore be desirable to treat the Resin fraction separately in a cost-effective manner.
TABLE 2DAOFEED(HC limited)RESINPITCHVOL-%100.0053.2114.7332.06WEIGHT-%100.0050.0015.0035.00API5.3714.22.9−6.1Sp. Gr.1.03380.97151.05261.1287S, wt-%4.273.035.095.69N, wppm0.3001Con Carbon, wt-%237.723.044.8C7 insols, wt-%6.860.020.119.5UOP K11.2711.5411.2210.92Ni, ppm242.014.459.6V, ppm945.230.2248.2
For applications where the only downstream hydroprocessing route is hydrocracking, the quality of the DAO is much more restrictive. Even with resin hydroprocessing, the hydroprocessed resin stream may not be suitable as VGO Hydrocracker feedstock. Therefore, further selective separation of the hydroprocessed resin stream would be beneficial to produce additional VGO Hydrocracking feedstock for those applications where hydrocracking is the downstream hydroprocessing route.